The reaction for the fluorination of perchloroethylene with HF in the gas phase in the presence of a catalyst is known. It generally results in the formation of 2,2-dichloro-1,1,1-trifluoroethane (123), 2-chloro-1,1,1,2-tetrafluoroethane (124) and pentafluoroethane (125) with 123 as the major product.
These compounds (denoted overall hereinbelow by the expression (“120 series”) can be used either as substitutes for chlorofluorocarbons (CFCs), in the fields of foams (blowing agents and insulators), aerosols (propellants) or refrigeration, or as intermediates in the synthesis of these substitutes. A search is currently underway for high performance processes for the industrial production of pentafluoroethane.
The document WO 92/16479 discloses a process for the manufacture of 2,2-dichloro-1,1,1-trifluoroethane (123), 2-chloro-1,1,1,2-tetrafluoroethane (124) and pentafluoroethane (125) by reacting perchloroethylene (PER) with HF in the gas phase in the presence of a catalyst comprising zinc fluoride supported on fluorinated alumina at a temperature of between 250 and 450° C. and with a contact time of between 0.1 and 60 sec. This document teaches the use preferably of an HF/PER molar ratio of between 3 and 10.
It is recommended, in order to obtain pentafluoroethane as the major product, to carry out the operation in two stages. Thus, the document EP 811 592 provides a two-stage process according to which PER is reacted with HF in the liquid phase in the presence of a catalyst and at a temperature of between 60 and 150° C., to give 123 and/or 1,1,2-trichloro-2,2-difluoroethane (122), and the 123 and/or 122 formed in the first stage is/are subsequently reacted in the vapour phase with HF in the presence of a catalyst and at a temperature of between 250 and 450° C.
Furthermore, this document teaches that, as the reaction for the fluorination of perchloroethylene to give pentafluoroethane is a highly exothermic reaction (28 kcal/mol), there are a number of problems posed by carrying it out in a single stage: the reaction is difficult to control, the catalyst decomposes and significant amounts of by-products are formed.